Generally, CRT displays include an automatic beam limiter (referred to as ABL hereinafter) for limiting CRT beam currents to less than a predetermined rate in order to prevent the radiation of X-ray and protect the CRT circuits.
Common ABL circuits are designed for detecting the beam current in a high-voltage generating circuit or a CRT cathode circuit and if the beam current exceeds a predetermined threshold, limiting the amplitude of a video signal.
Such a detecting process is implemented by a beam current detecting circuit mounted in a CRT driving circuit which drives video signals of R, G, and B colors before delivering to their respective CRT cathodes for display of an image, and adapted for detecting the beam current for each cathode and when the beam current exceeds a predetermined threshold, limiting the amplitude of its video signal. The arrangement and action of an automatic beam current limiting circuit in a color television receiver disclosed in Japanese Patent Laid-open Publication H5-276461 are now explained referring to FIG. 7.
As shown in FIG. 7, the beam current detecting circuit denoted at 50 which detects the beam currents of R, G, and B colors is interposed between the cathode of a CRT 53 and a video power amplifier 52 for driving the cathodes with their respective R, G, and B video signals. A contrast control circuit 51 is connected between an input video signal 10 and the video power amplifier 52. The input video signal 10 is fed from the contrast control circuit 51 to the video power amplifier 52 where it is amplified and then transmitted via the beam current detecting circuit 50 to the CRT 53. The contrast control circuit 51 has a control terminal 58 thereof for receiving a control voltage to control the contrast. When the control voltage at the control terminal 58 is decreased, contrast control circuit 51 lowers its video signal output thus causing the CRT drive video signals or beam currents to be reduced.
The beam current detecting circuit 50 delivers the video signals to the CRT 53 and also detects the beam currents of their respective colors. The beam currents are then filtered and resultant DC voltages are fed to an ABL control circuit 55.
When any of the DC voltages derived from their respective R, G, and B beam currents is greater than a reference voltage Vref in the ABL control circuit 55, one of three transistors Tr1(B), Tr2(G), and Tr3(R) in the ABL control circuit 55 is turned on so as to flow current through a resistor R1 and a resistor R2. As a result, the voltage at the control input terminal 58 of the contrast control circuit 51 is reduced causing the R, G, and B video signals to be reduced in amplitude so that the beam currents are controlled.
Another example of the conventional circuitry method of limiting the peak value of beam currents for prevention of blooming in an image with the help of an ABL circuit is also known as disclosed in Japanese Patent Laid-open Publication H5-191742 which is illustrated in FIG. 8A.
The arrangement and action of the peak brightness (or peak current) limiting circuit shown in FIG. 8A is explained in detail. A peak brightness limiting circuit 62 is provided upstream of a CRT 70 and a brightness signal processing circuit 67 which includes a video power amplifier for driving the CRT 70.
A video signal 10 is supplied via the peak brightness limiting circuit 62 to the brightness signal processing circuit 67 where it is amplified by the video power amplifier for driving the CRT 70.
An ABL control voltage generating circuit 72 is provided for detecting a beam current from the anode of the CRT 70 and delivering a corresponding ABL control voltage 74.
When the beam current from the CRT is great, representing a high level of brightness on the CRT 70, the ABL control voltage 74 of the ABL control voltage generating circuit 72 becomes lower. The ABL control voltage 74 is added with an output voltage of a contrast control voltage generating circuit 71 and fed to the brightness signal processing circuit 67 for attenuating the beam current. If the beam current of the CRT 70 is small, representing a low brightness level on the CRT, the sum of the voltage signals increases the beam current. The right half of FIG. 8A is the ABL circuit which comprises the CRT 70, the contrast control voltage generating circuit 71, the ABL control voltage generating circuit 72, a high-voltage generating circuit 73, and the brightness signal processing circuit 67 for detecting beam current from the anode of the CRT 70 and controlling the beam current.
The video signal 10 is supplied to an average picture level (referred to as APL hereinafter) detecting circuit 63 as well as the peak brightness limiting circuit 62. The APL detecting circuit 63 picks up and transmits an APL voltage to a comparator circuit 64. The ABL control voltage 74 is also fed to the comparator circuit 64.
The comparator circuit 64 examines the APL voltage and the ABL control voltage 74 and, when detecting an intermediate level between a high APL enabling ABL and a low APL disabling ABL (which may produce a dim image having regions of peak brightness), controls the peak brightness limiting circuit 62 to attenuate the gain of a high amplitude range of the video signal as shown in FIG. 8B. As the gain of the high amplitude range of the video signal has been attenuated, the beam current peak is controlled minimizing a blooming effect in an image.
It is known as a technological dilemma of CRT displays that problems may arise when the screen size or the horizontal scanning frequency is increased. For maintaining the brightness at an equal level on a larger size of the screen with the anode voltage remaining uniform, the beam current has to be increased corresponding to the ratio of enlargement of the screen. For increasing the horizontal scanning frequency, the frequency range of the video signal has to be proportionally increased thus demanding the use of a wider frequency range of the video signal processing circuit for driving the CRT. Accordingly, it is essential for increasing the screen size or the horizontal scanning frequency to employ the CRT driving signal of a high amplitude and a wide frequency range.
The conventional ABL circuit, where a detecting circuit is interposed between the video power amplifier and the CRT or connected ahead of the CRT, may be effective if the beam currents of three colors for the CRT are controlled separately. However, while the foregoing demand is involved, installation of the detecting signal in the video signal power circuit for handling the video signal of a high amplitude may increase a floating capacitance of the video signal power circuit thus reducing the frequency range characteristic of the CRT driving circuit.
Also, the output section of the video power amplifier for high amplitude and wide frequency range should employ as the power transistor a higher slew rate transistor which is unfavorable in the cost. It is known that the higher slew rate transistor in the video amplifier circuit enhances the frequency range characteristics when the amplitude of the video signal is small, allowing amplification of a wider frequency range and handling a higher amplitude of the video signal. It is understood, however, that such transistors, as capable of amplifying the wide frequency range of a high power signal passed between the video power amplifier and the CRT, are very expensive and may be commercially available with difficulty.
Furthermore, the conventional method in which the APL of video signals is detected and used for limiting the amplitude peak of the video signal to determine the peak value of beam currents at the preceding step of the video power amplifier for driving the cathode of a CRT is effective in the beam current limiting action for the CRT. It is true, however, that the linearity of screen brightness may be reduced and the detection of the R, G, and B beam currents is hardly conducted on the basis of each color due to the use of a CRT anode for the beam current detection.